Optical communication systems have continued to gain popularity in today's data transmission markets. Primarily because of their fast transmission speed, small size and relatively precise manufacture, Optical communication systems incorporating numerous optical devices and assemblies have become the systems of choice for technology companies desiring high-speed information transmission capabilities. Accordingly, as the transfer of information becomes one of the most valuable commodities in the world, optical device manufacturers are eager to develop further improvements in optoelectronic technology.
One area relating to optical devices that has seen significant improvement in recent years is the modulation of optical signals before transmission across an optical network. More specifically, Mach-Zehnder modulators (MZM) fabricated using III–V semiconductor materials have gained continued popularity for high-speed optical fiber communication systems. Compared with an older lithium niobate (LiNbO3) modulator, a III–V MZM is typically more compact, less expensive, and compatible with monolithic integration. Compared with a conventional electro-absorption modulator, a III–V MZM has zero or tunable chirp, broader optical bandwidth, and can modulate higher optical power. However, the electro-optic (EO) coefficient is relatively small for III–V materials. Due to this fact, the required modulation length for achieving low modulation voltage (Vn<5V) makes it difficult to achieve a wide operational bandwidth (e.g., 30–40 GHz). This compounds the challenge for III–V MZMs to achieve both low modulation voltage Vn and wide bandwidth for 40 Gb/s applications.
There are two major types of III–V MZMs. One type is based on the linear electro-optic effect (EO). Since the linear EO coefficient is small for III–V materials, modulation waveguide length >1 cm is typically needed to obtain a low modulation voltage Vn. However, such a relatively long modulation waveguide usually contains a large modulation loading capacitance (CL˜4 pF), which makes it practically impossible to achieve the desired wide bandwidth operation mentioned above with a lumped-element electrode. As a result, push-pull operation and traveling-wave electrodes having a microwave transmission line coupled to the dual optical waveguides are usually designed for this type of III–V MZM. The series push-pull design for the two arms in an MZM often cuts the loading capacitance in half, to CL˜2 pF for the same modulation length, yet still achieving the same modulation voltage Vn. The use of traveling-wave electrodes can further distribute the capacitance over the separately designed transmission lines, which lay in parallel with the dual optical waveguides.
In such a design, modulation length (and its capacitance) on the optical waveguides are segmented and periodically connected with conductive bridges to the traveling-wave electrodes as capacitive loading CL. The capacitive loading CL lowers the electrode (e.g., transmission line) impedance and the microwave propagation velocity through the modulator. If the lowered microwave impedance matches with the impedance of the external microwave source (usually about 50Ω), and the lowered microwave velocity matches with the optical group velocity, the microwaves and the modulated optical wave packet will co-propagate toward the same direction in pace. Thus, the modulation depth may be enhanced independent of microwave frequency and a wider operational bandwidth may be achieved.
Unfortunately, a typical III–V MZM using the push-pull scheme with traveling-wave electrodes still cannot safely meet the system requirement. The reported maximum performance for this kind of design is Vn=4.75 V, bandwidth ˜40 GHz, with an electrode length of 1.7 cm. The total optical waveguide length, including optical couplers and other passive waveguide lengths, total more than 2 cm. From these results, those who are skilled in the art understand that the operational bandwidth for the traveling-wave MZM is thus limited by the residual impedance mismatch, velocity mismatch, microwave loss and other parasitic parameters. Although increasing the modulation length of the MZM may help lower the modulation voltage Vn the operational bandwidth of the device will also be reduced.
Another type of III–V MZM uses multiple-quantum-well (MQW) as the active modulation layer based on quantum confined Stark effect (QCSE). This quadratic effect (which means that the index change is roughly proportional to the square of the change in electrical field) requires a relatively thin (e.g., 0.4 μm) active layer to provide a large electrical field. However, such a thin active layer typically leads to a large loading capacitance for the modulation waveguide (˜1 pF/mm).
Although the required modulation length in such a device is desirably short (e.g., 0.5–1.0 mm) for a low modulation voltage Vn, a relatively long electrode (>0.5 cm) is usually required if the above-mentioned push-pull traveling-wave design is used. In such a device, the total optical waveguide length will be close to 1 cm. Unfortunately, while achieving a much shorter length, a large optical loss (e.g., >20 dB) typically occurs for this type of device. Therefore, a MQW type III–V MZM is usually designed as a lumped-element device, thus making large operational bandwidth difficult to achieve. For example, a typical maximum bandwidth achieved for this type of lumped-element device is ˜15 GHz, with Vn˜4.5 V. As a result, the application of the MQW type III–V MZM is usually limited to 10 Gb/s optical communication systems.
Accordingly, what is needed in the art is an optical modulation waveguide device capable of high bandwidth operation, while maintaining a relatively short overall length and a low modulation voltage Vn.